In the beginning allow defenders to blocking out properly by penalizing attackers:
After the shot attacker must count to TWO then he is allowed to go for offensive rebound. This will give the defender enough time to concentrate on proper blocking-out!

When defenders become more successful, then this drill must be executed like in a real game!

Create own rules of playing 1:1 then block out! - Attacker shot on the spot, after 1,2... dribbles, etc...